Onegai Teacher is a pretty good anime, but it falls short from being anything more. If you're not expecting a masterpiece, it can be quite entertaining really. There's a good amount of humor and some fun characters to follow. The story is rather weak, but it's good enough.
The relationship between Mizuho and Kei just never seems to click. It never seems to stop being awkward, and that's really ashame. I just can't agree with some of the "choices" made in the series, and I know anyone in that position would not choose the path in this series. It feels stretched with no explanation of why. Mizuho makes even less sense in this fashion. I'm being quite vague, but I don't want to spoil things.
Frankly I think they just tried way too hard. Just when you think things might finally get on track, they're jerked back again and without warning, things take a sharp left turn. It barrows far too much from series like Ah! My Goddess and does a rather poor job at it. Even the name bugs me... Onegai Teacher? Couldn't they at least call it "Onegai Sensei"?
This isn't a series I'd recommend, as hard as that is for me to say about any anime. If you've watched all the other recent animes and you need something new, go for it. Otherwise, skip it. It's not horrible, in fact I had a rather good time watching it. Personally, I believe there's just better animes out there to choose from. Other folks have loved this anime though, so it's all a matter of personal preference.